Carrier recovery based demodulation

ABSTRACT

A demodulator which is arranged to demodulate a first signal with the aid of a second signal, comprises a first bandpass ( 30 ) filter arranged to recover the first signal ( 36 ) from a received signal ( 10 ) and a second bandpass filter ( 32 ) arranged to recover the second signal ( 30 ) from the received signal ( 10 ). Wherein the passband of the second bandpass filter ( 32 ) is substantially narrower than the passband of the first bandpass filter ( 30 ).

The present invention relates to a demodulator. The invention furtherrelates to an apparatus using such a demodulator and to a method fordemodulating. More particularly, the present invention relates todemodulators based on carrier recovery such as demodulators for TV-IFsignals, radio-IF and any other broadband IF system that make use of arecovered signal such as a carrier signal or a center frequency signalfor the demodulation.

Demodulators that are based on carrier recovery are generally known inthe art These demodulators comprise a single bandpass filter tosimultaneously recover the carrier and the information signal from areceived input signal. Such a demodulator is known from Taub-Schillingsecond edition, ISBN: 0-07-100313-4 on page 347, FIG. 8.1-A.

To this end, the demodulator is arranged to demodulate a first signalwith the aid of a second signal, the demodulator comprising:

-   -   a first bandpass filter arranged to recover the first signal        from a received signal; and    -   a second bandpass filter arranged to recover the second signal        from the received signal;

in which the passband of the second bandpass filter is substantiallynarrower than the passband of the first bandpass filter.

The invention is based on the insight that the use of a single bandpassfilter for the recovery of the first and second signals, may lead to anattenuation of at least 6 dB of the second signal due to the nyquistslope of the single bandpass filter. This may lead to an unreliablerecovery of the second signal, which is particularly problematic duringpoor receiving conditions. According to the present invention, thesecond signal for example a carrier signal or a center frequency signalcan be recovered by using an additional bandpass filter having a narrowpassband which is designed to recover only the second signal whilstrejecting the surrounding noise for as much as possible. The inventiontherefore provides two advantages. First of all the 6 dB attenuation ofthe second signal can be avoided. And second, due to the rejection ofthe surrounding noise, the Signal to Noise Ratio can even further beimproved. Obviously, the principle of noise rejection is generallyapplicable and will therefore provide the benefit of improving theSignal to Noise Ratio even for demodulators that do not suffer from theNyquist slope.

In an embodiment according to the present invention the demodulatorcomprises compensation means for minimizing a phase difference betweenthe recovered first signal and the recovered second signal. These phasedifferences may result in an impaired demodulation of the first signaland they are caused by differences in propagation delay between thefirst and second bandpass filter.

In a further embodiment according to the present invention, thecompensation means comprises a delay element that is arranged to delaythe recovered first signal. Herewith, the total delay of the firstsignal through the first band pass filter and the delay element equalsthe delay of the second signal through the second bandpass filter.

In an other embodiment according to the present invention thecompensation means comprises a phase shifter that is arranged to shift aphase of the recovered second signal, the phase shift being dependentupon the phase difference between a reference signal and the recoveredsecond signal. This embodiment has the advantage that it provides adynamic compensation of the phase differences which is not aimed at aspecific combination of used bandpass filters. Obviously, this makes thesolution a very flexible one. An embodiment of the phase shifter can befound on page 120 of: “Phaselock Techniques” by Floyd M. Gardner, 2^(nd)edition, Wiley & Sons, Inc. ISBN 0-471-04294-3.

In an embodiment according to the present invention, the compensationmeans comprises a selector that is arranged to select the referencesignal from at least two reference sources. This allows selection of asuitable source for providing the reference signal.

In an embodiment according to the present invention the selector is aprogrammable selector. This has the advantage that the demodulator can,depending on its operational state automatically select the mostconvenient or reliable source for providing the reference signal.

In another embodiment according to the present invention one of the atleast two sources is an image of a demodulated first signal which isstored in memory means. By using an image of the demodulated firstsignal instead of the demodulated first signal, the demodulation is nolonger being disturbed by the presence of noise in the demodulatedsignal when it is fed back to the phase shifter 50.

In an embodiment according to the present invention the memory meanscomprises an analogue to digital converter arranged to provide a digitalimage of the demodulated first signal which allows for a convenientstorage of the image in a digital format.

In an embodiment according to the present invention, the demodulatorfurther comprises a phase locked loop for stabilizing the recoveredsecond signal for further improving the quality of the demodulation.

In an other embodiment according the present invention, the secondrecovered signal is used for frequency down converting a third signalsuch as a further carrier signal. This embodiment is particularly suitedfor TV-IF systems wherein the third signal can be a sound carrier.

These and other aspects of the present invention will be elucidatedfurther by means of the following drawings.

FIG. 1 shows an embodiment of a demodulator according to the prior art.

FIG. 2 shows the nyquist slope of a bandpass filter.

FIG. 3 shows an embodiment of a demodulator according to presentinvention.

FIG. 4 shows an other embodiment of a demodulator according to thepresent invention wherein phase differences are compensated by using adelay element.

FIG. 5 shows a further embodiment of a demodulator according to thepresent invention wherein phase differences are compensated by using aphase shifter.

FIG. 6 shows a further embodiment of a demodulator according to thepresent invention wherein the recovered second signal such as a carriersignal is stabilized using a Phase Locked Loop.

FIG. 7 shows an embodiment according to the present invention whereinthe recovered second signal such as carrier signal is used to down-mix afurther signal.

FIG. 8 shows an apparatus comprising a demodulator according to thepresent invention.

FIG. 9 shows a flow chart for a method for carrier recovery baseddemodulation according to the present invention.

FIG. 1 shows a demodulator according to the prior art. Shown is a singlebandpass filter 12 for filtering the received signal 10. The filteredsignal 15 comprises both the recovered first and second signal which bymeans of example represent an information signal and a carrier signal.In FIG. 1 the information signal is demodulated in module 13 comprisinga mixer 14 and a PLL 16 which is designed to lock onto the frequency ofthe carrier signal.

FIG. 2 shows the filter characteristics of bandpass filter 12 in moredetail. Shown is a second signal 20 which falls within the nyquist slope22 of the filter. This will attenuate the second signal approximately 6dB and will therefore hamper demodulation of the first signalparticularly if the received signal 10 has a poor Signal to Noise Ratio.

FIG. 3 shows a demodulator according to the present invention comprisingbandpass filter 30 for recovering the first signal 36. The demodulatorfurther comprises bandpass filter 32 for recovering the second signal38. By means of example the first signal 36 represents an informationsignal comprising the information that needs to be demodulated whilstthe second signal 38 represents a carrier signal. The passband of filter32 is substantially narrower than the passband of filter 30. In additionthe passband of filter 32 is designed to optimally recover the carriersignal 38. This means that the noise surrounding the carrier signal 38is largely filtered out whilst the carrier signal itself is notattenuated by any Nyquist slope. This results in an improved carrier tonoise ratio.

If however the propagation delay of filters 30 and 32 are not equal toeach other, phase errors between the recovered information signal 36 andthe recovered carrier signal 38 could occur. Obviously, this will have adeteriorating effect on the signal quality of the demodulatedinformation signal. Therefore in FIG. 4, delay element 40 is added fordelaying the information signal so that the difference in propagationdelay between filters 30, 32 is compensated for.

FIG. 5 shows an embodiment wherein the differences in the propagationdelay between filters 30 and 32 are compensated by use of phase shifter.An embodiment of such a phase shifter can be found on page 120 of:“Phaselock Techniques” by Floyd M. Gardner, 2^(nd) edition, Wiley &Sons, Inc. ISBN 0-471-04294-3. The phase shift depends on the phasedifference between recovered carrier signal 38 and reference signal 51.This reference signal 51 can be the demodulated information signal 18 oran image of this demodulated information signal. The image of theinformation signal 18 is averaged out by means of low pass filter 37before it is stored in stored in memory means 35. The stored image canbe static, provided that the stored image is reliable. The main benefitof using a stored image, is that in this way the demodulation is nolonger disturbed by the presence of noise which may be comprised in thedemodulated information signal 18 when it is fed back to phase shifter50. The embodiment is further equipped with programmable selector 31 forselecting either the demodulated information signal 18 or the image ofthe demodulated signal 18 as the source for providing the referencesignal 51. Herewith, the demodulator can automatically select the mostsuitable source for providing the reference signal 51 for example duringstartup. In this case the selector 31 would select the demodulatedsignal 18. At the same time however, memory 35 will store an averagedimage of the demodulated signal 18. Once a reliable image is stored, theselector 31 may select memory means 35 for providing the referencesignal 51. Finally, this embodiment is particularly advantageous in thatit provides an automatic and adaptive compensation of the phasedifferences that are caused by the differences in propagation delaybetween filters 30 and 32.

FIG. 6 shows a further embodiment according to the present inventionwhich is for example used in TV-IF systems. In FIG. 6, the carriersignal 38 is stabilized using Phase Locked Loop 60 which comprises asingle oscillator such as a Voltage Controlled Oscillator (VCO) 62. ThePhase Locked Loop 60 further comprises mixer 64 and low pass filter 66.The VCO 62 will oscillate at the frequency of the recovered carriersignal. In this embodiment, the mixing signals for mixers 34 and 64 aregenerated using single oscillator 62 thereby achieving considerablepower and avoiding deteriorating effects like for example oscillatorpulling, which occurs when two oscillators are influencing each other.Therefore this embodiment is particularly suited for integration in anintegrated circuit.

FIG. 7 shows an other embodiment according to the present inventionwherein the frequency of the VCO 62 is used to down-convert a secondcarrier signal 75 which is also recovered from received signal 10 bymeans of second bandpass filter 70. The recovered carrier signal isdown-converted using mixer 71. This embodiment can advantageously beused in TV-IF systems wherein the second carrier could be a soundcarrier. Because of the absence of a nyquist slope, the embodiment doesnot suffer from AM to PM conversion i.e. there are no disturbingcross-talk effects between the video audio signals.

FIG. 8 shows an apparatus 88 comprising a RF tuner 80 arranged toreceive a particular RF signal 81 and a demodulator 82 for demodulatingthe received signal 81.

FIG. 9 shows a flow chart for carrier recovery based demodulationaccording to the present invention. In the first step S1, a receivedsignal is bandpass filtered by means of a first bandpass filter forrecovering the first signal from the received signal. In the second stepS2, the received signal is bandpass filtered by means of a secondbandpass filter for recovering the second signal from the receivedsignal. Finally, in the third step S3, the first signal is demodulatedby means of the second signal.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. All signal processing shown in theabove embodiments can be carried in the analogue domain and the digitaldomain. The word “comprising” does not exclude the presence of elementsor steps other than those listed in a claim. The word “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measures cannot be used to advantage.

1. Demodulator arranged to demodulate a first signal with the aid of asecond signal, the demodulator comprising: a first bandpass filterarranged to recover the first signal from a received signal; a secondbandpass filter arranged to recover the second signal from a receivedsignal, the passband of the second bandpass filter being substantiallynarrower than the passband of the first bandpass filter; andcompensation means for compensating phase error between the recoveredfirst and second signals, the compensation means comprising a phaseshifter that is arranged to shift a phase of the recovered first signal,the phase shift being dependent upon the phase difference between therecovered second signal and a reference signal, the compensation meansfurther comprising a selector that is arranged to select the referencesignal from at least two sources.
 2. Demodulator according to claim 1,wherein the compensation means comprises a delay element that isarranged to delay the recovered first signal.
 3. Demodulator accordingto claim 1, wherein the selector is a programmable selector. 4.Demodulator according to claim 1, wherein one of the at least twosources is a demodulated first signal.
 5. Demodulator according to claim1, wherein one of the at least two source is an image of a demodulatedfirst signal which is stored in memory means.
 6. Demodulator accordingto claim 5 wherein, the memory means comprises an analogue to digitalconverter arranged to provide a digital image of the demodulated firstsignal.
 7. Demodulator according to claim 1 wherein the demodulatorfurther comprises a phase locked loop for stabilizing the recoveredsecond signal.
 8. Demodulator according to claim 1 wherein the recoveredsecond signal is used for frequency down converting at least a thirdsignal.
 9. Demodulator according to claim 1 further comprising a mixerconnected to the first and second bandpass filters to mix the firstsignal and the second signal.
 10. Apparatus comprising a demodulator,the demodulator being arranged to demodulate a first signal with the aidof a second signal, the demodulator comprising: a first bandpass filterarranged to recover the first signal from a received signal; a secondbandpass filter arranged to recover the second signal from the receivedsignal, the passband of the second bandpass filter being substantiallynarrower than the passband of the first bandpass filter; andcompensation means for compensating phase error between the recoveredfirst and second signals, the compensation means comprising a phaseshifter that is arranged to shift a phase of the recovered first signal,the phase shift being dependent upon the phase difference between therecovered second signal and a reference signal, the compensation meansfurther comprising a selector that is arranged to select the referencesignal from at least two sources.
 11. Method for demodulating a firstsignal with the aid of a second signal the method comprising the stepsof: using a first bandpass filter for recovering the first signal from areceived signal; using a second bandpass filter having a substantiallynarrower passband than the first bandpass filter, for recovering thesecond signal from the received signal; and compensating phase errorbetween the recovered first and second signals, the compensatingincluding shifting a phase of the recovered first signal, the shiftingbeing dependent upon the phase difference between the recovered secondsignal and a reference signal, the compensating further includingselecting the reference signal from at least two sources.